Plastic cocrni-based medium-entropy alloy with 2.0 gpa-level ultra-high yield strength and preparation method thereof

ABSTRACT

The present disclosure belongs to the field of preparation of high-performance alloy materials, and specifically relates to a plastic CoCrNi-based medium-entropy alloy with 2.0 GPa-level ultra-high yield strength and a preparation method thereof. The alloy is prepared by melting and casting, homogenization treatment, solution heat treatment, cold deformation and aging heat treatment. After cold deformation and aging heat treatment, the prepared alloy has a dual heterogeneous microstructure due to the discontinuous precipitation of the strengthening phase and the incomplete recrystallization composition. The CoCrNi-based medium-entropy alloy of the present disclosure has ultra-high yield strength (2.0 GPa) and sufficient safety in use (uniform elongation of more than 8%), which can be processed into various forms of products, and has a wide range of applications in the production of fasteners used in the fields of aerospace, navigation, oil and gas, food processing, springs, non-magnetic components, and instrument parts.

CROSS REFERENCE TO RELATED APPLICATION

This patent application claims the benefit and priority of Chinese Patent Application No. 202111358940.4, filed on Nov. 17, 2021, the disclosure of which is incorporated by reference herein in its entirety as part of the present application.

TECHNICAL FIELD

The present disclosure belongs to the field of preparation of high-performance alloy materials, and specifically relates to a plastic CoCrNi-based medium-entropy alloy with 2.0 GPa-level ultra-high yield strength and a preparation method thereof.

BACKGROUND ART

Corrosion-resistant alloys have ultra-high yield strength, which can endow the structural alloys with excellent resistance to stress corrosion cracking under high stress conditions, so that the alloys can obtain good service performance in harsh environments. This high performance is especially suitable for aerospace fasteners, space shuttle structures, chemical processing, medical, cryogenic equipment, marine equipment, oil and gas, food processing, springs, non-magnetic components, instrument parts and other fields. Before the mid-1970s, the standard aerospace fasteners used H-11 alloy (AMS6408), which is a variant of hot work die steel. Under the premise of meeting the plasticity required for the safe use of fasteners (tensile elongation not less than 8%), it has a yield strength close to 1500 MPa, and can reach a strength level of 1800 MPa at the highest, which can meet the performance requirements of fasteners in aerospace and other fields at that time. However, this series of alloys is essentially an iron-based alloy. In order to achieve a high level of strength, the chromium content is controlled below 5.0 mass %, so the series of alloys have poor corrosion resistance. In order to improve the corrosion resistance of fasteners, it is usually necessary to use a low-brittle cadmium fluoroborate process for electroplating. For fasteners that require a higher level of strength, vacuum deposition of cadmium or even diffusion of nickel-cadmium is required, which greatly increases manufacturing cost. Even so, this kind of coating has a very poor protective effect on stress corrosion, and it is easy to cause pitting corrosion locally and become a crack nucleation source of fracture, which brings great safety hazards. In addition, the fasteners treated in this way are not suitable for the seawater environment, and the seawater will damage the cadmium plating layer of the protective layer, which greatly limits the application of this kind of material in the field of fasteners.

In order to improve the performance of fasteners, in the 1980s, the fastener industry began to use nickel-based super alloys with better corrosion resistance to develop a new generation of materials for high-performance fasteners. The first choice was GH4169 alloy with the most outstanding hardening ability to achieve the requirements for high strength of the material, the selected strengthening and toughening process is cold deformation with controllable strain and low temperature aging heat treatment. However, while satisfying the fastener plasticity (tensile elongation not less than 8%), the yield strength only reaches the level of 1500 MPa, which only reaches the lower limit of the strength level of fasteners made of AMS6408 alloy. Therefore, this alloy system can only replace AMS6408 to prepare low-end fasteners of 1500 MPa.

In order to meet the needs of higher-end development in military fields such as aerospace and high-performance missiles, cobalt-based alloys with higher mechanical properties have been developed one after another. The representative alloys are multiphase MP35N and MP159. Among them, the MP35N alloy uses martensitic transformation produced by cold deformation and low-temperature aging to strengthen and toughen. The special strengthening and toughening mechanism allows the alloy to obtain a tensile strength of more than 2000 MPa while retaining good toughness. However, this material has poor continuous strain hardening ability, premature necking in the deformation process, and poor uniform deformation ability, which brings hidden dangers to the application. Generally, the yield strength is at the level of 2000 MPa, and the uniform tensile elongation is only 1-2%. Moreover, the usable temperature of the alloy is relatively low.

MP159 series and gold are the alloys used in the highest grades of aerospace fasteners today and are the most widely used cobalt-based multiphase alloys since the 1980s. This kind of alloy is strengthened and toughened by the martensitic transformation (ε lamella) formed by cold deformation and low temperature aging and the structure of Ni₃X (x=Ti, Nb, Al) ordered η phase. Due to the formation of ε lamellae, the control requirements of the plastic deformation process in cold deformation are very strict, and the processability of the material is poor. At the same time, under the premise of satisfying the plasticity required in engineering, the yield strength obtained due to the addition of Fe is lower, which is only 1800 MPa level. In addition, the continuous strain hardening ability of the multiphase material is still poor, and necking occurs earlier in the deformation process, which is manifested by a high shrinkage of area after fracture (generally above 35%), which makes the safety of the alloy poor. At the same time, the usable temperature needs to be improved.

Taken together, the above alloy systems mainly rely on deformation strengthening and precipitation strengthening to achieve toughness, and there is a significant trade-off effect between “toughness-strength”. Therefore, under the premise of maintaining a certain degree of plasticity, the ability to increase strength, especially yield strength, is limited.

SUMMARY

In view of the shortcomings of the prior art, the present disclosure proposes a CoCrNi-based medium-entropy corrosion-resistant alloy with ultra-high yield strength (2.0 GPa) and sufficient safety in use (uniform elongation of more than 8%) and a preparation method thereof. Under the premise that the uniform elongation rate is greater than 8%, the strength of this alloy reaches the level of MP35N series alloy (yield strength reaches 2.0 GPa).

A plastic CoCrNi-based medium-entropy alloy with ultra-high yield strength provided by the present disclosure comprises the following components in atomic percentage (at. %): Cr: 14-25%, Ni: 25-35%, Al: 4-7%, Ti: 4-7%, Mo: 0-5%, and the balance is Co.

Wherein, it preferably comprises the following components in atomic percentage: Cr: 17-22%, Ni: 28-30%, Al: 5-6%, Ti: 5-6%, Mo: 0-2%, and the balance is Co.

The present disclosure also provides a method for preparing the plastic CoCrNi-based medium-entropy alloy with ultra-high yield strength, wherein comprising the following steps:

-   -   (1) Preparing an alloy according to the above atomic percentage         and melting into an ingot;     -   (2) Homogenizing the ingot to form a homogenized casting;     -   (3) Performing solution heat treatment on the homogenized         casting to obtain an alloy casting with FCC single phase;     -   (4) Performing cold deformation with a deformation of 70-90% on         the casting after solution heat treatment;     -   (5) Performing aging heat treatment on the cold deformed         material to obtain the plastic CoCrNi-based medium-entropy alloy         with 2.0 GPa-level ultra-high yield strength.

In some embodiments, in step (2), the homogenization treatment is performing heat preservation at 1000-1200° C. for 12-24 h, after the completion of the heat preservation, quenching to room temperature.

In some embodiments, in step (3), the solution heat treatment is performing heat preservation at 1150-1200° C. for 2-8 h, after the completion of the heat preservation, quenching to room temperature.

In some embodiments, in step (4), the cold deformation is cold rolling, or rotary swaging and/or drawing at room temperature.

In some embodiments, in step (5), the temperature of the aging heat treatment is 600-800° C., and the time is 4-28 h.

In the present disclosure, the composition of the CoCrNi-based medium-entropy alloy is precisely designed to obtain low stacking fault energy and strong precipitation ability, and through a special cold mechanical deformation and aging process, the recrystallization and the discontinuous precipitation kinetics of the strengthening phase are controlled to obtain a dual heterostructure with a high density nano-strengthening phase distributed in the crystal matrix with a clear structural gradient. Therefore, under the premise that the back stress effectively improves the yield strength, an additional strain hardening effect is generated through local uneven deformation during the deformation process, thereby achieving comprehensive strengthening and toughness during the deformation process, resulting in the mutual coupling of back stress strengthening and toughening and precipitation strengthening and toughening, and endowing the alloy with super high plastic performance indexes. In addition, the low alloying of Mo in the present disclosure can produce a strong solid solution strengthening effect and contribute to the improvement of the yield strength.

The present disclosure provides a series of CoCrNi-based mid-entropy alloys with ultra-high mechanical properties and a preparation method thereof. The alloy is a corrosion-resistant alloy and through the reasonable design of the composition and the preparation process of Co—Cr—Ni—Al—Ti(Mo), the grain structure of matrix and the precipitation dual inhomogeneous structure can be obtained, so that the alloy can obtain excellent comprehensive mechanical properties in a large temperature range, and has sufficient use safety (uniform elongation of more than 8%). The alloy can be processed into various forms of products and has a wide range of applications in the production of fasteners used in the fields of aerospace, navigation, oil and gas, food processing, springs, non-magnetic components, and instrument parts. The use of Co, Ni, Cr, Al, Ti and a small amount of Mo makes the alloy price moderate, the material preparation process is relatively simple, and the industrialization investment is low.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will be further described in detail below in conjunction with the drawings and embodiments:

FIG. 1 shows the structure of uneven crystal grains obtained after aging heat treatment of the CoCrNi-based medium-entropy alloy with ultra-high mechanical properties in Example 1 of the present disclosure;

FIG. 2 shows the mechanical properties of the CoCrNi-based medium-entropy alloy with ultra-high mechanical properties in the tensile process at room temperature in Example 1 of the present disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS Example 1

(1) An alloy with a composition of (Co₄₀Ni₃₀Cr₂₀Al₅Ti₅)_(0.995)Mo_(0.5) (at. %) was prepared, where the subscript of each element was the atomic percentage of the element, the alloy was melted into a 5 Kg ingot through a vacuum induction furnace;

(2) The ingot was homogenized at 1200° C./12 h and quenched to room temperature to obtain a homogenized casting;

(3) The homogenized casting was subjected to solution heat treatment at 1200° C./2 h and quenched to room temperature to obtain a single-phase alloy casting with face-centered cubic structure (FCC);

(4) The alloy casting after solution heat treatment was subjected to cold deformation (cold rolling or rotary swaging at room temperature) with a deformation of 80%;

(5) The cold-deformed part was subjected to aging heat treatment at 650° C./24 h to obtain the plastic CoCrNi-based medium-entropy alloy (sheet or bar) with ultra-high yield strength.

The microstructure of the obtained CoCrNi-based medium-entropy alloy is shown in FIG. 1 . It can be seen that in the microstructure of the alloy, there are both nano-scale recrystallized matrix grain structure and micron-scale unrecrystallized structure, forming a strong heterogeneous structure.

The obtained alloy sample was stretched at a tensile rate of 10⁻³ s⁻¹. The results are shown in FIG. 2 . At room temperature, the tensile ductility of the sample reaches 10%, the yield strength reaches 2000 MPa, and the tensile strength reaches 2018 MPa.

Example 2

(1) An alloy with a composition of (Co₄₃Ni₃₀Cr₁₅Al₆Ti₆ (at. %) was prepared, and melted into a 5 Kg ingot through a vacuum induction furnace;

(2) The ingot was homogenized at 1000° C./20 h and quenched to room temperature to obtain a homogenized casting;

(3) The homogenized casting was subjected to solution heat treatment at 1200° C./3 h and quenched to room temperature to obtain a single-phase alloy casting with face-centered cubic structure (FCC);

(4) The alloy casting after solution heat treatment was subjected to cold deformation (cold rolling or rotary swaging at room temperature) with a deformation of 75%;

(5) The cold-deformed part was subjected to aging heat treatment at 650° C./28 h to obtain the plastic CoCrNi-based medium-entropy alloy (sheet or bar) with ultra-high yield strength.

The obtained alloy sample was stretched at a tensile rate of 10⁻³ s⁻¹. At room temperature, the tensile ductility of the sample reaches 8%, the yield strength reaches 1950 MPa, and the tensile strength reaches 1980 MPa.

Example 3

(1) An alloy with a composition of (Co₄₀Ni₃₀Cr₂₀Al₅Ti₅)_(0.985)Mo_(1.5) (at. %) was prepared, and melted into a 5 Kg ingot through a vacuum induction furnace;

(2) The ingot was homogenized at 1100° C./24 h and quenched to room temperature to obtain a homogenized casting;

(3) The homogenized casting was subjected to solution heat treatment at 1200° C./4 h and quenched to room temperature to obtain a single-phase alloy casting with face-centered cubic structure (FCC);

(4) The alloy casting after solution heat treatment was subjected to cold deformation (cold rolling or rotary swaging at room temperature) with a deformation of 75%;

(5) The cold-deformed part was subjected to aging heat treatment at 700° C./12 h to obtain the plastic CoCrNi-based medium-entropy alloy (sheet or bar) with ultra-high yield strength.

The obtained alloy sample was stretched at a tensile rate of 10⁻³ s⁻¹. At room temperature, the tensile ductility of the sample reaches 9%, the yield strength reaches 2001 MPa, and the tensile strength reaches 2012 MPa.

Example 4

(1) An alloy with a composition of (Co₄₃Ni₃₀Cr₁₅Al₆Ti₆)_(0.995)Mo_(0.5) (at. %) was prepared, and melted into a 5 Kg ingot through a vacuum induction furnace;

(2) The ingot was homogenized at 1200° C./12 h and quenched to room temperature to obtain a homogenized casting;

(3) The homogenized casting was subjected to solution heat treatment at 1150° C./8 h and quenched to room temperature to obtain a single-phase alloy casting with face-centered cubic structure (FCC);

(4) The alloy casting after solution heat treatment was subjected to cold deformation (cold rolling or rotary swaging at room temperature) with a deformation of 80%;

(5) The cold-deformed part was subjected to aging heat treatment at 650° C./18 h to obtain the plastic CoCrNi-based medium-entropy alloy (sheet or bar) with ultra-high yield strength.

The obtained alloy sample was stretched at a tensile rate of 10⁻³ s⁻¹. At room temperature, the tensile ductility of the sample reaches 10%, the yield strength reaches 1999 MPa, and the tensile strength reaches 2018 MPa.

Example 5

(1) An alloy with a composition of (Co₄₃Ni₃₀Cr₁₅Al₆Ti₆)_(0.99)Mo₁ (at. %) was prepared, and melted into a 5 Kg ingot through a vacuum induction furnace;

(2) The ingot was homogenized at 1200° C./12 h and quenched to room temperature to obtain a homogenized casting;

(3) The homogenized casting was subjected to solution heat treatment at 1200° C./4 h and quenched to room temperature to obtain a single-phase alloy casting with face-centered cubic structure (FCC);

(4) The alloy casting after solution heat treatment was subjected to cold deformation (cold rolling or rotary swaging at room temperature) with a deformation of 80%;

(5) The cold-deformed part was subjected to aging heat treatment at 700° C./18 h to obtain the plastic CoCrNi-based medium-entropy alloy (sheet or bar) with ultra-high yield strength.

The obtained alloy sample was stretched at a tensile rate of 10⁻³ s⁻¹. At room temperature, the tensile ductility of the sample was reached 8%, the yield strength was reached 1990 MPa, and the tensile strength was reached 2018 MPa.

Example 6

(1) An alloy with a composition of (Co₄₃Ni₃₀Cr₁₅Al₆Ti₆)_(0.985)Mo_(1.5) (at. %) was prepared, and melted into a 5 Kg ingot through a vacuum induction furnace;

(2) The ingot was homogenized at 1200° C./24 h and quenched to room temperature to obtain a homogenized casting;

(3) The homogenized casting was subjected to solution heat treatment at 1200° C./4 h and quenched to room temperature to obtain a single-phase alloy casting with face-centered cubic structure (FCC);

(4) The alloy casting after solution heat treatment was subjected to cold deformation (cold rolling or rotary swaging at room temperature) with a deformation of 80%;

(5) The cold-deformed part was subjected to aging heat treatment at 725° C./10 h to obtain the plastic CoCrNi-based medium-entropy alloy (sheet or bar) with ultra-high yield strength.

The obtained alloy sample was stretched at a tensile rate of 10⁻³ s⁻¹. At room temperature, the tensile ductility of the sample reaches 8%, the yield strength reaches 1990 MPa, and the tensile strength reaches 2018 MPa.

Example 7

(1) An alloy with a composition of (Co₄₀Ni₃₀Cr₂₀Al₅Ti₅)_(0.99)Mo₁ (at. %) was prepared, and melted into a 5 Kg ingot through a vacuum induction furnace;

(2) The ingot was homogenized at 1200° C./18 h and quenched to room temperature to obtain a homogenized casting;

(3) The homogenized casting was subjected to solution heat treatment at 1200° C./4 h and quenched to room temperature to obtain a single-phase alloy casting with face-centered cubic structure (FCC);

(4) The alloy casting after solution heat treatment was subjected to cold deformation (cold rolling or rotary swaging at room temperature) with a deformation of 80%;

(5) The cold-deformed part was subjected to aging heat treatment at 700° C./18 h to obtain the plastic CoCrNi-based medium-entropy alloy (sheet or bar) with ultra-high yield strength.

The obtained alloy sample was stretched at a tensile rate of 10⁻³ s⁻¹. At room temperature, the tensile ductility of the sample reaches 8%, the yield strength reaches 2003 MPa, and the tensile strength reaches 2020 MPa.

Example 8

(1) An alloy with a composition of (Co₄₀Ni₃₀Cr₂₀Al₅Ti₅)_(0.98)Mo₂ (at. %) was prepared, and melted into a 5 Kg ingot through a vacuum induction furnace;

(2) The ingot was homogenized at 1200° C./24 h and quenched to room temperature to obtain a homogenized casting;

(3) The homogenized casting was subjected to solution heat treatment at 1200° C./4 h and quenched to room temperature to obtain a single-phase alloy casting with face-centered cubic structure (FCC);

(4) The alloy casting after solution heat treatment was subjected to cold deformation (cold rolling or rotary swaging at room temperature) with a deformation of 80%;

(5) The cold-deformed part was subjected to aging heat treatment at 780° C./6 h to obtain the plastic CoCrNi-based medium-entropy alloy (sheet or bar) with ultra-high yield strength.

The obtained alloy sample was stretched at a tensile rate of 10⁻³ s⁻¹. At room temperature, the tensile ductility of the sample reaches 8%, the yield strength reaches 1999 MPa, and the tensile strength reaches 2010 MPa.

Example 9

(1) An alloy with a composition of Co₄₀Ni₃₀Cr₂₀Al₅Ti₅ (at. %) was prepared, and melted into a 5 Kg ingot through a vacuum induction furnace;

(2) The ingot was homogenized at 1200° C./24 h and quenched to room temperature to obtain a homogenized casting;

(3) The homogenized casting was subjected to solution heat treatment at 1200° C./4 h and quenched to room temperature to obtain a single-phase alloy casting with face-centered cubic structure (FCC);

(4) The alloy casting after solution heat treatment was subjected to cold deformation (cold rolling or rotary swaging at room temperature) with a deformation of 80%;

(5) The cold-deformed part was subjected to aging heat treatment at 600° C./28 h to obtain the plastic CoCrNi-based medium-entropy alloy (sheet or bar) with ultra-high yield strength.

The obtained alloy sample was stretched at a tensile rate of 10⁻³ s⁻¹. At room temperature, the tensile ductility of the sample reaches 8%, the yield strength reaches 1990 MPa, and the tensile strength reaches 1980 MPa. 

1. A plastic CoCrNi-based medium-entropy alloy with 2.0 GPa-level ultra-high yield strength, wherein comprising the following components in atomic percentage: Cr: 14-25%, Ni: 25-35%, Al: 4-7%, Ti: 4-7%, Mo: 0-5%, and the balance is Co.
 2. The plastic CoCrNi-based medium-entropy alloy with 2.0 GPa-level ultra-high yield strength according to claim 1, wherein comprising the following components in atomic percentage: Cr: 17-22%, Ni: 28-30%, Al: 5-6%, Ti: 5-6%, Mo: 0-2%, and the balance is Co.
 3. The plastic CoCrNi-based medium-entropy alloy with 2.0 GPa-level ultra-high yield strength of claim 1 with a composition selected from (Co₄₀Ni₃₀Cr₂₀Al₅Ti₅)_(0.995)Mo_(0.5) (at. %), (Co₄₃Ni₃₀Cr₁₅Al₆Ti₆ (at. %), (Co₄₀Ni₃₀Cr₂₀Al₅Ti₅)_(0.985)Mo_(1.5) (at. %), (Co₄₃Ni₃₀Cr₁₅Al₆Ti₆)_(0.995)Mo_(0.5) (at. %), (Co₄₃Ni₃₀Cr₁₅Al₆Ti₆)_(0.99)Mo₁ (at. %), (Co₄₃Ni₃₀Cr₁₅Al₆Ti₆)_(0.985)Mo_(1.5) (at. %), (Co₄₀Ni₃₀Cr₂₀Al₅Ti₅)_(0.99)Mo₁ (at. %), (Co₄₀Ni₃₀Cr₂₀Al₅Ti₅)_(0.98)Mo₂ (at. %) and (Co₄₀Ni₃₀Cr₂₀Al₅Ti₅)_(0.98)Mo₂ (at. %).
 4. The plastic CoCrNi-based medium-entropy alloy with 2.0 GPa-level ultra-high yield strength of claim 1 having a uniform elongation of more than 8%.
 5. The plastic CoCrNi-based medium-entropy alloy with 2.0 GPa-level ultra-high yield strength of claim 1 formed into a fastener.
 6. A method for preparing the plastic CoCrNi-based medium-entropy alloy with 2.0 GPa-level ultra-high yield strength according to claim 1, comprising the following steps: (1) Preparing an alloy according to the atomic percentage of claim 1 and melting into an ingot; (2) Homogenizing the ingot to form a homogenized casting; (3) Performing solution heat treatment on the homogenized casting to obtain an alloy casting with FCC single phase; (4) Performing cold deformation with a deformation of 70-90% on the casting after solution heat treatment; (5) Performing aging heat treatment on the cold deformed material to obtain the plastic CoCrNi-based medium-entropy alloy with 2.0 GPa-level ultra-high yield strength.
 7. The method for preparing the plastic CoCrNi-based medium-entropy alloy with 2.0 GPa-level ultra-high yield strength according to claim 6, wherein in step (2), the homogenization treatment is performing heat preservation at 1000-1200° C. for 12-24 h, after the completion of the heat preservation, quenching to room temperature.
 8. The method for preparing the plastic CoCrNi-based medium-entropy alloy with 2.0 GPa-level ultra-high yield strength according to claim 6, wherein in step (3), the solution heat treatment is performing heat preservation at 1150-1200° C. for 2-8 h, after the completion of the heat preservation, quenching to room temperature.
 9. The method for preparing the plastic CoCrNi-based medium-entropy alloy with 2.0 GPa-level ultra-high yield strength according to claim 6, wherein in step (4), the cold deformation is cold rolling, or rotary swaging and/or drawing at room temperature.
 10. The method for preparing the plastic CoCrNi-based medium-entropy alloy with 2.0 GPa-level ultra-high yield strength according to claim 6, wherein the temperature of the aging heat treatment is 600-800° C., and the time is 4-28 h.
 11. A method for preparing the plastic CoCrNi-based medium-entropy alloy with 2.0 GPa-level ultra-high yield strength, comprising the following steps: (1) Preparing an alloy according to the atomic percentage of claim 1 and melting into an ingot; (2) Homogenizing the ingot to form a homogenized casting; (3) Performing solution heat treatment on the homogenized casting to obtain an alloy casting with FCC single phase; (4) Performing cold deformation with a deformation of 70-90% on the casting after solution heat treatment; (5) Performing aging heat treatment on the cold deformed material to obtain the plastic CoCrNi-based medium-entropy alloy with 2.0 GPa-level ultra-high yield strength.
 12. The method for preparing the plastic CoCrNi-based medium-entropy alloy with 2.0 GPa-level ultra-high yield strength according to claim 11, wherein in step (2), the homogenization treatment is performing heat preservation at 1000-1200° C. for 12-24 h, after the completion of the heat preservation, quenching to room temperature.
 13. The method for preparing the plastic CoCrNi-based medium-entropy alloy with 2.0 GPa-level ultra-high yield strength according to claim 11, wherein in step (3), the solution heat treatment is performing heat preservation at 1150-1200° C. for 2-8 h, after the completion of the heat preservation, quenching to room temperature.
 14. The method for preparing the plastic CoCrNi-based medium-entropy alloy with 2.0 GPa-level ultra-high yield strength according to claim 11, wherein in step (4), the cold deformation is cold rolling, or rotary swaging and/or drawing at room temperature.
 15. The method for preparing the plastic CoCrNi-based medium-entropy alloy with 2.0 GPa-level ultra-high yield strength according to claim 11, wherein the temperature of the aging heat treatment is 600-800° C., and the time is 4-28 h.
 16. The method for preparing the plastic CoCrNi-based medium-entropy alloy with 2.0 GPa-level ultra-high yield strength according to claim 11, wherein the deformation is 80%.
 17. The method for preparing the plastic CoCrNi-based medium-entropy alloy with 2.0 GPa-level ultra-high yield strength according to claim 11, wherein the alloy has a composition of (Co₄₀Ni₃₀Cr₂₀Al₅Ti₅)_(0.995)Mo_(0.5) (at. %) and wherein the ingot was homogenized at 1200° C. for 12 h, the homogenized casting was subjected to solution heat treatment at 1200° C. for 12 h and quenched at room temperature, and the cold-deformed part was subjected to aging heat treatment at 650 C for 24 h.
 18. The method for preparing the plastic CoCrNi-based medium-entropy alloy with 2.0 GPa-level ultra-high yield strength according to claim 11, wherein the alloy has a composition selected from (Co₄₀Ni₃₀Cr₂₀Al₅Ti₅)_(0.995)Mo_(0.5) (at. %), (Co₄₃Ni₃₀Cr₁₅Al₆Ti₆ (at. %), (Co₄₀Ni₃₀Cr₂₀Al₅Ti₅)_(0.985)Mo_(1.5) (at. %), (Co₄₃Ni₃₀Cr₁₅Al₆Ti₆)_(0.995)Mo_(0.5) (at. %), (Co₄₃Ni₃₀Cr₁₅Al₆Ti₆)_(0.99)Mo₁ (at. %), (Co₄₃Ni₃₀Cr₁₅Al₆Ti₆)_(0.985)Mo_(1.5) (at. %), (Co₄₀Ni₃₀Cr₂₀Al₅Ti₅)_(0.99)Mo₁ (at. %), (Co₄₀Ni₃₀Cr₂₀Al₅Ti₅)_(0.98)Mo₂ (at. %) and (Co₄₀Ni₃₀Cr₂₀Al₅Ti₅)_(0.98)Mo₂ (at. %).
 19. The method for preparing the plastic CoCrNi-based medium-entropy alloy with 2.0 GPa-level ultra-high yield strength according to claim 11 further comprising forming a fastener from said plastic CoCrNi-based medium-entropy alloy. 